SARS 2014
MINIMISING SODAR EXTRANEOUS NOISE
THROUGH DESIGN WITH
A 3D-PRINTED ULTRASONIC SCALE MODEL
Adrien Chabbey, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland
Stuart Bradley, Physics Department, University of Auckland
Fernando Porté-Agel, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland
SARS 2014
MOTIVATION
Sodars need good acoustic shielding
Testing on full-scale systems is difficult
A scale model approach allows for
“bench testing” of many designs
For example, scaling a Metek sodar by
21:1 can be achieved using 40 kHz
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SARS 2014
3D PRINTER SCALE MODEL
Metek PCS-2000/24 LP sodar operating at 1.9 kHz
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Phased array sodar is operating at 40 kHz
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SARS 2014
LAB MEASUREMENTS
microphone array
transmitter
b
x
z
x
y
floor
Convenient to use distances in units of wavelength 
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SARS 2014
COMPARISON WITH MODEL
D = 9.5
Bare array
H = 6.8
A Kirchhoff Integral Theorem
model has been developed for all
comparisons with measurements.
Measured (solid line) and
computed (dashed line) polar
patterns are shown.
With baffle
x
z
Noise affects results below -30 dB.
x
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y
x-z plane
x'-z plane
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SARS 2014
D = 9.5
TILTED BEAM
H = 6.8
x
z
Main beam
x
y
Unwanted side lobe
Measured (solid line) and calculated (dashed line) beam pattern of
the bare phased array for a beam tilted in the x-z plane.
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Measured (solid line) and calculated (dashed line) beam pattern of
the phased array with baffle for a beam tilted in the x-z plane.
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SARS 2014
LEAKAGE DUE TO DIFFRACTION
The baffle intercepts a little of the energy from
the first side lobe (at around 22) as well as the
side lobes which are at lower elevations.
0
Normalised intensity [dB]
-10
But this interception is accompanied by
diffraction. Increasing H, in an effort to cut out
more transverse sound, can actually make
things worse!
-20
No baffle
-30
One partial solution is to modulate the top of
the baffle with “thnadners”.
-40
D
Baffle
-50
Thnadners
-60
0
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15
30
45
60
Zenith angle [deg]
75
90
H
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SARS 2014
LOW ELEVATION (0-20) INTENSITY
Vertical
beam
Side lobes
Main lobe
More complex. Any improvement?
Tilted
beam
Side lobes
D = 9.5 , H = 6.8  baffle. No thnadners
Main lobe
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With 40 thnadners of height 2.15
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SARS 2014
OPTIMISATION: NO THNADNERS
D
H/D = constant produces a minimum.
This corresponds to the baffle rim being at the interference
pattern notch at 34, which means that the power
intersecting the rim and resulting in diffraction is reduced.
H
But optimisation for the tilted beam is much less effective. This
probably accounts for the main reason sodars are so noisy.
Vertical beam
H
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Tilted beam
H
D
D
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SARS 2014
PEAK INTENSITY 0-20 ELEVATION
Tilted beam
There is a periodicity with the
number of thnadners (at N =
24, 28, 32,…). This corresponds
to an integral number of
thnadners on each of the 4
sides.
Thnadner height ( units)
Thnadner height ( units)
Vertical beam
Number of thnadners
The thnadners are acting as a
diffraction grating which has
a different angular periodicity
to that of the array.
Number of thnadners
Each little triangle is a thnadner
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Optimum thnadner height is
2 but not very heightdependent.
Only about 2 dB improvement
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SARS 2014
MEAN INTENSITY IN 0-20 ANNULUS
Thnadner height ( units)
Averaging around a full circle gives
an idea of average effect for any
site.
A shallow minimum occurs at
thnadner height = 2.3 and number
of thnadners = 37. This compares
closely with the Metek design of 2.15
and 40.
Number of thnadners
But addition of thnadners only gives
about 1 dB improvement in annulusaveraged intensity.
Each little triangle is a thnadner
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SARS 2014
SUMMARY
Sodars can be bench-tested using models scaled by wavelength.
This allows for full-functionality testing in the lab.
3D printing allows many models to be tested at very low cost.
We investigate a 21:1 scale model of a commercial Metek sodar.
For the basic baffle, without thnadners, the key parameter is baffle height to width
ratio H/D: increasing both H and D by 50%, for example, gives no reduction in lateral
sound.
H/D should be chosen so that the baffle rim is at an angle of low intensity. This has long
been known by trial and error, but until now the sensitivity to this has been unknown.
Thnadners act as a diffraction grating of different angular periodicity to the array
pattern. This causes the array pattern to be broken up. Energy is dispersed into other
angles, not necessarily giving overall improvement.
Thnadners spacing (diffraction grating ruling width) is much more significant than
thnadner height.
Thnadners can give around 2 dB reduction in laterally-propagating sound.
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